IKKbeta activation is sufficient for RANK-independent osteoclast differentiation and osteolysis

Nuclear Factor-Kappa B Regulation of Osteoclastogenesis and Osteoblastogenesis

Maintenance of skeletal integrity requires the coordinated activity of multinucleated bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoclasts form resorption lacunae on bone surfaces in response to cytokines by fusion of precursor cells. Osteoblasts are derived from mesenchymal precursors and lay down new bone in resorption lacunae during bone remodeling. Nuclear factorkappa B (NF-κB) signaling regulates osteoclast and osteoblast formation and is activated in osteoclast precursors in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which can also control osteoblast formation through RANK-RANKL reverse signaling in osteoblast precursors. RANKL and some pro-inflammatory cytokines, including tumor necrosis factor (TNF), activate NF-κB signaling to positively regulate osteoclast formation and functions. However, these cytokines also limit osteoclast and osteoblast formation through NF-κB signaling molecules, including TNF receptor-associated factors (TRAFs). TRAF6 mediates RANKL-induced osteoclast formation through canonical NF-κB signaling. In contrast, TRAF3 limits RANKL- and TNF-induced osteoclast formation, and it restricts transforming growth factor β (TGFβ)-induced inhibition of osteoblast formation in young and adult mice. During aging, neutrophils expressing TGFβ and C-C chemokine receptor type 5 (CCR5) increase in bone marrow of mice in response to increased NF-κB-induced CC motif chemokine ligand 5 (CCL5) expression by mesenchymal progenitor cells and injection of these neutrophils into young mice decreased bone mass. TGFβ causes degradation of TRAF3, resulting in decreased glycogen synthase kinase-3β/β-catenin-mediated osteoblast formation and age-related osteoporosis in mice. The CCR5 inhibitor, maraviroc, prevented accumulation of TGFβ+/CCR5+ neutrophils in bone marrow and increased bone mass by inhibiting bone resorption and increasing bone formation in aged mice. This paper updates current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast and osteoblast formation and activation with a focus on the role of TRAF3 signaling, which can be targeted therapeutically to enhance bone mass.

Interleukin-23 Regulates Inflammatory Osteoclastogenesis via Activation of CLEC5A(+) Osteoclast Precursors

OBJECTIVE

To investigate the role of interleukin-23 (IL-23) in pathologic bone remodeling in inflammatory arthritis.

METHODS

In this study we investigated the role of IL-23 in osteoclast differentiation and activation using in vivo gene transfer techniques in wild-type and myeloid DNAX-activation protein 12-associating lectin-1 (MDL-1)-deficient mice, and by performing in vitro and in vivo osteoclastogenesis assays using spectral flow cytometry, micro-computed tomography analysis, Western blotting, and immunoprecipitation.

RESULTS

Herein, we show that IL-23 induces the expansion of a myeloid osteoclast precursor population and supports osteoclastogenesis and bone resorption in inflammatory arthritis. Genetic ablation of C-type lectin domain family member 5A, also known as MDL-1, prevents the induction of osteoclast precursors by IL-23 that is associated with bone destruction, as commonly observed in inflammatory arthritis. Moreover, osteoclasts derived from the bone marrow of MDL-1-deficient mice showed impaired osteoclastogenesis, and MDL-1-/- mice had increased bone mineral density.

CONCLUSION

Our data show that IL-23 signaling regulates the availability of osteoclast precursors in inflammatory arthritis that could be effectively targeted for the treatment of inflammatory bone loss in inflammatory arthritis.

OPA1 drives macrophage metabolism and functional commitment via p65 signaling

Macrophages are essential players for the host response against pathogens, regulation of inflammation and tissue regeneration. The wide range of macrophage functions rely on their heterogeneity and plasticity that enable a dynamic adaptation of their responses according to the surrounding environmental cues. Recent studies suggest that metabolism provides synergistic support for macrophage activation and elicitation of desirable immune responses; however, the metabolic pathways orchestrating macrophage activation are still under scrutiny. Optic atrophy 1 (OPA1) is a mitochondria-shaping protein controlling mitochondrial fusion, cristae biogenesis and respiration; clear evidence shows that the lack or dysfunctional activity of this protein triggers the accumulation of metabolic intermediates of the TCA cycle. In this study, we show that OPA1 has a crucial role in macrophage activation. Selective Opa1 deletion in myeloid cells impairs M1-macrophage commitment. Mechanistically, Opa1 deletion leads to TCA cycle metabolite accumulation and defective NF-κB signaling activation. In an in vivo model of muscle regeneration upon injury, Opa1 knockout macrophages persist within the damaged tissue, leading to excess collagen deposition and impairment in muscle regeneration. Collectively, our data indicate that OPA1 is a key metabolic driver of macrophage functions.

Osteoblast lineage cells and periodontal ligament fibroblasts regulate orthodontic tooth movement that is dependent on Nuclear Factor-kappa B (NF-kB) activation

OBJECTIVES

To investigate the role of NF-κB in osteoblast lineage cells and periodontal ligament (PDL) fibroblasts during orthodontic tooth movement (OTM).

MATERIALS AND METHODS

Transgenic mice that expressed a dominant negative mutant of the inhibitor of kB kinase (IKK-DN) with lineage specific expression in osteoblastic cells and PDL fibroblasts driven by a response element in the collagen1α1 promoter and matched wild-type (WT) mice were examined. A 10-12 g force was applied by a NiTi coil and maintained for 5 or 12 days. OTM distance, PDL width, and bone volume fraction were measured using micro computed tomography. Osteoclast numbers were counted in tartrate-resistant acid phosphatase-stained sections. Activation of nuclear factor kappa B (NF-kB) was assessed by nuclear localization of p65, and the receptor activator of nuclear factor-κB ligand (RANKL) was measured by immunofluorescence and compared to control specimens with no orthodontic force.

RESULTS

OTM-induced NF-kB activation (p65 nuclear localization) in WT mice was largely blocked in transgenic (TG) mice. OTM was significantly reduced in the TG mice compared to WT mice along with reduced osteoclastogenesis, narrower PDL width, higher bone volume fraction, and reduced RANKL expression.

CONCLUSIONS

Osteoblast lineage cells and PDL fibroblasts are key contributors to alveolar bone remodeling in OTM through IKKβ dependent NF-κB activation.

Different Cell and Tissue Behavior of Micro-/Nano-Tubes and Micro-/Nano-Nets Topographies on Selective Laser Melting Titanium to Enhance Osseointegration

Background and Purpose

Micro-/nano-tubes (TNTs) and micro-/nano-nets (TNNs) are the common and sensible choice in the first step of combined modifications of titanium surface for further functionalization in the purpose of extended indications and therapeutic effect. It is important to recognize the respective biologic reactions of these two substrates for guiding a biologically based first-step selection.

Materials and Methods

TNTs were produced by anodic oxidation and TNNs were formed by alkali-heat treatment. The original selective laser melting (SLM) titanium surface was set as control. Surface characterization was evaluated by scanning electron microscopy, surface roughness, and water contact angle measurements. Osteoclastogenesis and osteogenesis were measured. MC3T3-E1 cells and RAW 264.7 cells were used for in vitro assay in terms of adhesion, proliferation, and differentiation. In vivo assessments were taken on Beagle dogs with micro-CT and histological analysis.

Results

TNN and TNT groups performed decreased roughness and increased hydrophilicity compared with SLM group. For biological detections, the highest ALP activity and osteogenesis-related genes expression were observed in TNT group followed by TNN group (P <0.05). Interestingly, when it comes to the osteoclastogenesis, TNNs displayed lowest TRAP activity and osteoclastogenesis-related genes expression and TNTs were lower than SLM but higher than TNNs (P <0.05). BV/TV around implants was highest in TNT group after 4 weeks (P <0.05). HE, ALP and TRAP staining showed that osteogenic and osteoclastic activity around TNTs were both higher than TNNs (P <0.05).

Conclusion

TNNs and TNTs have dual advantages in promotion of osteogenesis and inhibition of osteoclastogenesis. Furthermore, TNNs showed better capability in inhibiting osteoclast activity while TNTs facilitated stronger osteogenesis. Our results implied that TNT substrates would take advantage in early application after implantation, while diseases with inappropriate osteoclast activity would prefer TNN substrates, which will guide a biologically based first-step selection on combined modification for different clinical purposes.

Linear ubiquitin assembly complex regulates lung epithelial-driven responses during influenza infection

Influenza A virus (IAV) is among the most common causes of pneumonia-related death worldwide. Pulmonary epithelial cells are the primary target for viral infection and replication and respond by releasing inflammatory mediators that recruit immune cells to mount the host response. Severe lung injury and death during IAV infection result from an exuberant host inflammatory response. The linear ubiquitin assembly complex (LUBAC), composed of SHARPIN, HOIL-1L, and HOIP, is a critical regulator of NF-κB-dependent inflammation. Using mice with lung epithelial-specific deletions of HOIL-1L or HOIP in a model of IAV infection, we provided evidence that, while a reduction in the inflammatory response was beneficial, ablation of the LUBAC-dependent lung epithelial-driven response worsened lung injury and increased mortality. Moreover, we described a mechanism for the upregulation of HOIL-1L in infected and noninfected cells triggered by the activation of type I IFN receptor and mediated by IRF1, which was maladaptive and contributed to hyperinflammation. Thus, we propose that lung epithelial LUBAC acts as a molecular rheostat that could be selectively targeted to modulate the immune response in patients with severe IAV-induced pneumonia.

All-trans retinoic acid increases NF-κB activity in PMA-stimulated THP-1 cells upon unmethylated CpG challenge by enhancing cell surface TLR9 expression

An active metabolite of vitamin A, all-trans retinoic acid (ATRA), is known to exert immunomodulatory functions. This study investigates the possible immune potentiating effect of ATRA on NF-κB activity in human monocytic THP-1 cells after exposure to unmethylated CpG DNA ODN2006. We observed that challenge with ODN2006 significantly enhanced the NF-κB activity of PMA-differentiated THP-1 cells. ATRA synergistically enhanced NF-κB activity of cells, in a concentration- and time-dependent manner. The enhanced NF-κB activity of PMA-differentiated THP-1 cells after ODN2006 challenge was dependent on the RAR/RXR pathway. To determine the mechanism involved in increasing in the NF-κB activity of stimulated THP-1 cells, we examined the effects of PMA and ATRA on the expression of TLR9 (a receptor of ODN2006) in THP-1 cells. PMA treatment significantly enhanced both the intracellular and cell surface expression of TLR9, while ATRA alone showed no effect. However, ATRA synergistically enhanced the cell surface TLR9 expression of PMA-differentiated cells. To determine whether the ATRA-enhanced NF-κB activity is due to the enhanced cell surface TLR9 expression, we examined NF-κB activity after treatment with anti-TLR9 blocking antibody. Results revealed that the anti-TLR9 antibody treatment almost completely reverses the ATRA-enhanced NF-κB activity, suggesting that ATRA enhances NF-κB activity through upregulation of the cell surface TLR9 expression in PMA-differentiated and unmethylated CpG challenged THP-1 cells.

Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

Intrinsic Restriction of TNF-Mediated Inflammatory Osteoclastogenesis and Bone Resorption

TNF (Tumor necrosis factor) is a pleiotropic cytokine that plays an important role in immunity and inflammatory bone destruction. Homeostatic osteoclastogenesis is effectively induced by RANKL (Receptor activator of nuclear factor kappa-B ligand). In contrast, TNF often acts on cell types other than osteoclasts, or synergically with RANKL to indirectly promote osteoclastogenesis and bone resorption. TNF and RANKL are members of the TNF superfamily. However, the direct osteoclastogenic capacity of TNF is much weaker than that of RANKL. Recent studies have uncovered key intrinsic mechanisms by which TNF acts on osteoclast precursors to restrain osteoclastogenesis, including the mechanisms mediated by RBP-J signaling, RBP-J and ITAM (Immunoreceptor tyrosine-based activation motif) crosstalk, RBP-J mediated regulatory network, NF-κB p100, IRF8, and Def6. Some of these mechanisms, such as RBP-J and its mediated regulatory network, uniquely and predominantly limit osteoclastogenesis mediated by TNF but not by RANKL. As a consequence, targeting RBP-J activities suppresses inflammatory bone destruction but does not significantly impact normal bone remodeling or inflammation. Hence, discovery of these intrinsic inhibitory mechanisms addresses why TNF has a weak osteoclastogenic potential, explains a significant difference between RANKL and TNF signaling, and provides potentially new or complementary therapeutic strategies to selectively treat inflammatory bone resorption, without undesirable effects on normal bone remodeling or immune response in disease settings.

Attenuation of NF-κB in Intestinal Epithelial Cells Is Sufficient to Mitigate the Bone Loss Comorbidity of Experimental Mouse Colitis

Skeletal abnormalities are common comorbidities of inflammatory bowel disease (IBD). Patients suffering from IBD, including ulcerative colitis and Crohn's disease, present with skeletal complications. However, the mechanism underpinning IBD-associated bone loss remains vague. Intestinal inflammation generates an inflammatory milieu at the intestinal epithelium that leads to dysregulation of mucosal immunity through gut-residing innate lymphoid cells (ILCs) and other cell types. ILCs are recently identified mucosal cells considered as the gatekeeper of gut immunity and their function is regulated by intestinal epithelial cell (IEC)-secreted cytokines in response to the inflammatory microenvironment. We first demonstrate that serum as well as IECs collected from the intestine of dextran sulfate sodium (DSS)-induced colitis mice contain high levels of inflammatory and osteoclastogenic cytokines. Mechanistically, heightened inflammatory response of IECs was associated with significant intrinsic activation of NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) in IECs and increased frequency of ILC1, ILC3, and myeloid osteoclast progenitors. Validating the central role of IEC-specific NF-κB activation in this phenomenon, conditional expression of constitutively active inhibitor kappa B kinase 2 (IKK2) in IECs in mice recapitulates the majority of the cellular, inflammatory, and osteolytic phenotypes observed in the chemically induced colitis. Furthermore, conditional deletion of IKK2 from IECs significantly attenuated inflammation and bone loss in DSS-induced colitis. Finally, using the DSS-induced colitis model, pharmacologic inhibition of IKK2 was effective in reducing frequency of ILC1 and ILC3 cells, attenuated circulating levels of inflammatory cytokines, and halted colitis-associated bone loss. Our findings identify IKK2 in IECs as viable therapeutic target for colitis-associated osteopenia.

Does TNF Promote or Restrain Osteoclastogenesis and Inflammatory Bone Resorption?

Chronic inflammation is one of the most evident and common pathological conditions leading to deregulated osteoclastogenesis and bone remodeling. Tumor necrosis factor (TNF) as a pleiotropic cytokine plays a key role, not only in inflammation, but also in bone erosion in diseases associated with bone loss. TNF can stimulate the proliferation of osteoclast precursors and, in most conditions, act together with other cytokines and growth factors such as receptor activator of nuclear factor (NF)-[kappa]B ligand (RANKL), interleukin-6, and transforming growth factor beta to synergistically promote osteoclast formation and bone resorption in vivo. A longstanding enigma in the field is why TNF alone is not able to induce osteoclast differentiation as effectively as the same superfamily member RANKL, a physiological master osteoclastogenic cytokine. Recent studies have highlighted several lines of evidence showing the intrinsic mechanisms through RBP-J, NF-[kappa]B p100/TNF receptor-associated factor 3, or interferon regulatory factor-8 that restrain TNF-induced osteoclast differentiation and bone resorption. These feedback inhibitory mechanisms driven by TNF shed light into the current paradigm of osteoclastogenesis and would provide novel therapeutic implications on controlling inflammatory bone resorption.

Magnesium lithospermate B inhibits titanium particles-induced osteoclast formation by c-fos and inhibiting NFATc1 expression

Purpose: Titanium particle-induced osteolysis is one of the important causes of aseptic loosening of artificial joints. Previous studies have shown the potential of natural compounds in preventing Ti particle-induced bone resorption. In this study, we observed the effects of magnesium lithospermate B (MLB) on titanium particle-induced osteoclast activity in vitro. Materials and Methods: RAW264.7 cells were treated with titanium particles (0.1 mg/mL) in the presence or absence of MLB (200 nmol/L). We evaluated the osteoclast formation, bone pits formation and tartrate-resistant acid phosphatase 5b (Tracp5b) levels. Reverse transcription polymerase chain reaction (RT-PCR) and Western blot were used to evaluate osteoclast differentiation-related genes (TRAF6, NFATc1, and c-fos) and protein expression. Results: The number of osteoclasts, pit formation and Tracp5b levels were all the group treated with titanium particles compared to the control group (all p < 0.05). Titanium particles also promoted the expression of the TRAF6, NFATc1 and c-fos genes and protein expression. MLB significantly abolished the titanium particle-enhanced osteoclast and pits formation, and Traf6, NFATc1, and c-fos expression. Conclusions: Our data demonstrated that MLB can suppress titanium-induced osteoclast activity via inhibiting c-fos and NFATc1 expression.

Pharmacological Inhibition of NFκB Reduces Prostate Cancer Related Osteoclastogenesis In Vitro and Osteolysis Ex Vivo

NFκB is implicated in cancer and bone remodelling, and we have recently reported that the verified NFκB inhibitor Parthenolide (PTN) reduced osteolysis and skeletal tumour growth in models of metastatic breast cancer. Here, we took advantage of in vitro and ex vivo bone cell and organ cultures to study the effects of PTN on the ability of prostate cancer cells and their derived factors to regulate bone cell activity and osteolysis. PTN inhibited the in vitro growth of a panel of human, mouse and rat prostate cancer cells in a concentration-dependent manner with a varying degree of potency. In prostate cancer cell-osteoclast co-cultures, the rat Mat-Ly-Lu, but not human PC3 or mouse RM1-BT, enhanced RANKL stimulated osteoclast formation and PTN reduced these effects without affecting prostate cancer cell viability. In the absence of cancer cells, PTN reduced the support of Mat-Ly-Lu conditioned medium for the adhesion and spreading of osteoclast precursors, and survival of mature osteoclasts. Pre-exposure of osteoblasts to PTN prior to the addition of conditioned medium from Mat-Ly-Lu cells suppressed their ability to support the formation of osteoclasts by inhibition of RANKL/OPG ratio. PTN enhanced the ability of Mat-Ly-Lu derived factors to increase calvarial osteoblast differentiation and growth. Ex vivo, PTN enhanced bone volume in calvaria organ-Mat-Ly-Lu cell co-culture, without affecting Mat-Ly-Lu viability or apoptosis. Mechanistic studies in osteoclasts and osteoblasts confirmed that PTN inhibit NFκB activation related to derived factors from Mat-Ly-Lu cells. Collectively, these findings suggest that pharmacological inhibition of the skeletal NFκB signalling pathway reduces prostate cancer related osteolysis, but further studies in the therapeutic implications of NFκB inhibition in cells of the osteoblastic lineage are needed.

miR-338-3p regulates osteoclastogenesis via targeting IKKβ gene

This study determined the effects of miR-338-3p on osteoclast (OC) differentiation and activation. The change levels of miR-338-3p in differentiated OCs were investigated by microRNA microarray assay and quantitative real-time PCR analysis. The effects of miR-338-3p on the differentiation and activation of OCs were determined by tartrate-resistant acid phosphatase staining resorption activity assay and Western blot. Target genes of miR-338-3p were identified by target gene prediction and dual-luciferase reporter gene detection assay as well as Western blot. Results showed that miR-338-3p was markedly downregulated in differentiated OCs. miR-338-3p could inhibit the formation and absorption activity of OCs. Western blot showed that miR-338-3p could influence the change levels of OC differentiation-related proteins. Dual-luciferase reporter gene detection assay and Western blot both showed that miR-338-3p directly targeted IKKβ gene. In conclusion, miR-338-3p may affect the formation and activity of OCs by targeting the IKKβ gene.

Cell Type-Specific Roles of NF-κB Linking Inflammation and Thrombosis

The transcription factor NF-κB is a central mediator of inflammation with multiple links to thrombotic processes. In this review, we focus on the role of NF-κB signaling in cell types within the vasculature and the circulation that are involved in thrombo-inflammatory processes. All these cells express NF-κB, which mediates important functions in cellular interactions, cell survival and differentiation, as well as expression of cytokines, chemokines, and coagulation factors. Even platelets, as anucleated cells, contain NF-κB family members and their corresponding signaling molecules, which are involved in platelet activation, as well as secondary feedback circuits. The response of endothelial cells to inflammation and NF-κB activation is characterized by the induction of adhesion molecules promoting binding and transmigration of leukocytes, while simultaneously increasing their thrombogenic potential. Paracrine signaling from endothelial cells activates NF-κB in vascular smooth muscle cells and causes a phenotypic switch to a “synthetic” state associated with a decrease in contractile proteins. Monocytes react to inflammatory situations with enforced expression of tissue factor and after differentiation to macrophages with altered polarization. Neutrophils respond with an extension of their life span—and upon full activation they can expel their DNA thereby forming so-called neutrophil extracellular traps (NETs), which exert antibacterial functions, but also induce a strong coagulatory response. This may cause formation of microthrombi that are important for the immobilization of pathogens, a process designated as immunothrombosis. However, deregulation of the complex cellular links between inflammation and thrombosis by unrestrained NET formation or the loss of the endothelial layer due to mechanical rupture or erosion can result in rapid activation and aggregation of platelets and the manifestation of thrombo-inflammatory diseases. Sepsis is an important example of such a disorder caused by a dysregulated host response to infection finally leading to severe coagulopathies. NF-κB is critically involved in these pathophysiological processes as it induces both inflammatory and thrombotic responses.

Pharmacological Inhibition of the Skeletal IKKβ Reduces Breast Cancer-Induced Osteolysis

IKKβ has previously been implicated in breast cancer bone metastasis and bone remodelling. However, the contribution of IKKβ expressed by bone cells of the tumour microenvironment to breast cancer-induced osteolysis has yet to be investigated. Here, we studied the effects of the verified selective IKKβ inhibitors IKKβ^III or IKKβ^V on osteoclast formation and osteoblast differentiation in vitro and in vivo, human and mouse breast cancer cells' support for osteoclast formation and signalling in vitro and osteolysis ex vivo and in immunocompetent mice after supracalvarial injection of human MDA-MB-231 conditioned medium or intra-cardiac injection of syngeneic 4T1 breast cancer cells. Pre-treatment with IKKβ^III or IKKβ^V prior to exposure to tumour-derived factors from human and mouse breast cancer cell lines protected against breast cancer-induced osteolysis in two independent immunocompetent mouse models of osteolysis and the ex vivo calvarial bone organ system. Detailed functional and mechanistic studies showed that direct inhibition of IKKβ kinase activity in osteoblasts and osteoclasts was associated with significant reduction of osteoclast formation, enhanced osteoclast apoptosis and reduced the ability of osteoblasts to support osteoclastogenesis in vitro. When combined with previous findings that suggest NFκB inhibition reduces breast cancer tumorigenesis and metastasis our present findings have an important clinical implication on raising the possibility that IKKβ inhibitors, as bone anabolics, osteoclast inhibitors as well as anti-metastatic agents, may have advantages over anti-osteoclasts agents in the treatment of both skeletal and non-skeletal complications associated with metastatic breast cancer.

Regulation of breast cancer induced bone disease by cancer-specific IKKβ

NFκB is implicated in breast cancer bone metastasis and skeletal remodelling. However, the role of IKKβ, a key component of the canonical NFκB pathway, in the regulation of breast cancer osteolytic metastasis has not been investigated. Here, we describe the cancer-specific contribution of IKKβ to bone metastasis, skeletal tumour growth and osteolysis associated with breast cancer. IKKβ is highly expressed in invasive breast tumours and its level of expression was higher in patients with bone metastasis. IKKβ overexpression in parental MDA-MD-231 breast cancer cells, promoted mammary tumour growth but failed to convey osteolytic potential to these cells in mice. In contrast, IKKβ overexpression in osteotropic sub-clones of MDA-MB-231 cells with differing osteolytic phenotypes increased incidence of bone metastasis, exacerbated osteolysis and enhanced skeletal tumour growth, whereas its knockdown was inhibitory. Functional and mechanistic studies revealed that IKKβ enhanced the ability of osteotropic MDA-MB-231 cells to migrate, increase osteoclastogenesis, and to inhibit osteoblast differentiation via a mechanism mediated, at least in part, by cytoplasmic sequestering of FoxO3a and VEGFA production. Thus, tumour-selective manipulation of IKKβ and its interaction with FoxO3a may represent a novel strategy to reduce the development of secondary breast cancer in the skeleton.

Optimized Extract from Corylopsis coreana Uyeki (Hamamelidaceae) Flos Inhibits Osteoclast Differentiation

Osteoporosis is a metabolic disorder that decreases the stability against fractures of the spine, femur, and radius by weakening the strength and integrity of bones. Receptor activator of nuclear factor-kappa B ligand signaling ultimately activated nuclear factor-activated T cells c1, a major transcription factor for osteoclast formation. This study researched the effects of Corylopsis coreana (C. coreana) Uyeki flos extracts on the antiosteoclastic potential of macrophages and the phytochemicals contained therein. The alcoholic extract of C. coreana Uyeki flos inhibited the differentiation of osteoclast. We carried out the experiments of the pattern of differentiation of osteoclasts based on the alcoholic percentage of extracts. Among them, 80% alcoholic extract showed the highest inhibitory effect. The alcoholic extract was composed of phytochemicals such as bergenin, quercetin, and quercitrin. This extract inhibited not only mRNA expression levels of NFATc1, osteoclast-associated receptor (OSCAR), cathepsin K, and tartrate-resistant acid phosphatase (TRAP), but also the translational expression of NFATc1. The inhibitory effect for osteoclast differentiation of the alcoholic extract was confirmed using the resorption pit assay. This is the first scientific report of the antiosteoclastic effects of C. coreana Uyeki flos extract, which can be applied therapeutically for the treatment of osteoporosis.

TRAF2 in osteotropic breast cancer cells enhances skeletal tumour growth and promotes osteolysis

NFκB plays an important role in inflammation and bone remodelling. Tumour necrosis factor receptor associated factor 2 (TRAF2), a key component of NFκB signalling, has been identified as an oncogene, but its role in the regulation of breast cancer osteolytic metastasis remains unknown. Here, we report that stable overexpression of TRAF2 in parental and osteotropic sub-clones of human MDA-MB-231 (MDA-231) breast cancer cells increased cell growth and motility in vitro, whereas TRAF2 knockdown was inhibitory. In vivo, TRAF2 overexpression in the parental MDA-231-P cells enhanced tumour growth after orthotopic injection into the mammary fat pad of mice but failed to promote the metastasis of these cells to bone. In contrast, overexpression of TRAF2 in osteotropic MDA-231-BT cells increased skeletal tumour growth, enhanced osteoclast formation and worsened osteolytic bone loss after intra-tibial injection in mice. Mechanistic and functional studies in osteotropic MDA-231-BT and osteoclasts revealed that upregulation of TRAF2 increased the ability of osteotropic MDA-231-BT cells to migrate and to enhance osteoclastogenesis by a mechanism dependent, at least in part, on NFκB activation. Thus, the TRAF2/NFκB axis is implicated in the regulation of skeletal tumour burden and osteolysis associated with advanced breast cancer.

Beyond the vicious cycle: The role of innate osteoimmunity, automimicry and tumor-inherent changes in dictating bone metastasis

Bone metastasis is a fatal consequence of a subset of solid malignancies that fail to respond to conventional therapies. While a myriad of factors contribute to osteotropism and disseminated cell survival and outgrowth in bone, efforts to inhibit tumor cell growth in the bone-metastatic niche have largely relied on measures that disrupt the bi-directional interactions between bone resident and tumor cells. However, the targeting of isolated stromal interactions has proven ineffective to date in inhibiting bone-metastatic progression and patient mortality. Osteoimmune regulation is now emerging as a critical determinant of metastatic growth in the bone microenvironment. While this has highlighted the importance of innate immune populations in dictating the temporal development of overt bone metastases, the osteoimmunological processes that underpin tumor cell progression in bone remain severely underexplored. Along with tumor-intrinsic alterations that occur specifically within the bone microenvironment, innate osteoimmunological crosstalk poses an exciting area of future discovery and therapeutic development. Here we review current knowledge of the unique exchange that occurs between bone resident cells, innate immune populations and tumor cells that leads to the establishment of a tumor-permissive milieu.

RANK-Independent Osteoclast Formation and Bone Erosion in Inflammatory Arthritis

OBJECTIVE

Proinflammatory molecules promote osteoclast-mediated bone erosion by up-regulating local RANKL production. However, recent evidence suggests that combinations of cytokines, such as tumor necrosis factor (TNF) plus interleukin-6 (IL-6), induce RANKL-independent osteoclastogenesis. The purpose of this study was to better understand TNF/IL-6-induced osteoclast formation and to determine whether RANK is absolutely required for osteoclastogenesis and bone erosion in murine inflammatory arthritis.

METHODS

Myeloid precursors from wild-type (WT) mice or mice with either germline or conditional deletion of Rank, Nfatc1, Dap12, or Fcrg were treated with either RANKL or TNF plus IL-6. Osteoprotegerin, anti-IL-6 receptor (anti-IL-6R), and hydroxyurea were used to block RANKL, the IL-6R, and cell proliferation, respectively. Clinical scoring, histologic assessment, micro-computed tomography, and quantitative polymerase chain reaction (qPCR) were used to evaluate K/BxN serum-transfer arthritis in WT and RANK-deleted mice. Loss of Rank was verified by qPCR and by osteoclast cultures.

RESULTS

TNF/IL-6 generated osteoclasts in vitro that resorbed mineralized tissue through a pathway dependent on IL-6R, NFATc1, DNAX-activation protein 12, and cell proliferation, but independent of RANKL or RANK. Bone erosion and osteoclast formation were reduced, but not absent, in arthritic mice with inducible deficiency of RANK. TNF/IL-6, but not RANKL, induced osteoclast formation in bone marrow and synovial cultures from animals deficient in Rank. Multiple IL-6 family members (IL-6, leukemia inhibitory factor, oncostatin M) were up-regulated in the synovium of arthritic mice.

CONCLUSION

The persistence of bone erosion and synovial osteoclasts in Rank-deficient mice, and the ability of TNF/IL-6 to induce osteoclastogenesis, suggest that more than one cytokine pathway exists to generate these bone-resorbing cells in inflamed joints.

Calcium Supplement Derived from Gallus gallus domesticus Promotes BMP-2/RUNX2/SMAD5 and Suppresses TRAP/RANK Expression through MAPK Signaling Activation

The present study evaluated the effects of a calcium (Ca) supplement derived from Gallus gallus domesticus (GD) on breaking force, microarchitecture, osteogenic differentiation and osteoclast differentiation factor expression in vivo in Ca-deficient ovariectomized (OVX) rats. One percent of Ca supplement significantly improved Ca content and bone strength of the tibia. In micro-computed tomography analysis, 1% Ca supplement attenuated OVX- and low Ca-associated changes in bone mineral density, trabecular thickness, spacing and number. Moreover, 1% Ca-supplemented diet increased the expression of osteoblast differentiation marker genes, such as bone morphogenetic protein-2, Wnt3a, small mothers against decapentaplegic 1/5/8, runt-related transcription factor 2, osteocalcin and collagenase-1, while it decreased the expression of osteoclast differentiation genes, such as thrombospondin-related anonymous protein, cathepsin K and receptor activator of nuclear factor kappa B. Furthermore, 1% Ca-supplemented diet increased the levels of phosphorylated extracellular signal-regulated kinase and c-Jun N-terminal kinase. The increased expression of osteoblast differentiation marker genes and activation of mitogen-activated protein kinase signaling were associated with significant increases in trabecular bone volume, which plays an important role in the overall skeletal strength. Our results demonstrated that 1% Ca supplement inhibited osteoclastogenesis, stimulated osteoblastogenesis and restored bone loss in OVX rats.

Strontium inhibits titanium particle-induced osteoclast activation and chronic inflammation via suppression of NF-κB pathway

Wear-particle-induced chronic inflammation and osteoclastogenesis have been identified as critical factors of aseptic loosening. Although strontium is known to be involved in osteoclast differentiation, its effect on particle-induced inflammatory osteolysis remains unclear. In this study, we investigate the potential impact and underling mechanism of strontium on particle-induced osteoclast activation and chronic inflammation in vivo and in vitro. As expected, strontium significantly inhibited titanium particle-induced inflammatory infiltration and prevented bone loss in a murine calvarial osteolysis model. Interestingly, the number of mature osteoclasts decreased after treatment with strontium in vivo, suggesting osteoclast formation might be inhibited by strontium. Additionally, low receptor activator of nuclear factor-κB ligand (RANKL), tumor necrosis factor-α, interleukin-1β, interleukin-6 and p65 immunochemistry staining were observed in strontium-treatment groups. In vitro, strontium obviously decreased osteoclast formation, osteoclastogenesis-related gene expression, osteoclastic bone resorption and pro-inflammatory cytokine expression in bone-marrow-derived macrophages in a dose-dependent manner. Furthermore, we demonstrated that strontium impaired osteoclastogenesis by blocking RANKL-induced activation of NF-κB pathway. In conclusion, our study demonstrated that strontium can significantly inhibit particle-induced osteoclast activation and inflammatory bone loss by disturbing the NF-κB pathway, and is an effective therapeutic agent for the treatment of wear particle-induced aseptic loosening.

Cytoplasmic hnRNPK interacts with GSK3β and is essential for the osteoclast differentiation

Osteoclast differentiation is a complex and finely regulated physiological process that involves a variety of signaling pathways and factors. Recent studies suggested that the Ser9 phosphorylation of Glycogen synthase kinase-3β (GSK3β) is required for the osteoclast differentiation. However, the precise underlying mechanism remains unclear. We have previously identified the heterogeneous nuclear ribonucleoprotein K (hnRNPK) as a putative GSK3β interactor. In the present study, we demonstrate that, during the RANKL-induced osteoclast differentiation, the PI3K/Akt-mediated Ser9 phosphorylation of GSK3β provokes the nuclear-cytoplasmic translocation of hnRNPK in an ERK-dependent manner, enhancing the cytoplasmic co-localization and interaction of GSK3β and hnRNPK. We show that hnRNPK is essential for the osteoclast differentiation, and is involved in several reported functions of GSK3β, including the activation of NF-κB, the expression of NFATc1, and the acetylation of tubulin, all known to be critical for osteoclast differentiation and functions. We find that hnRNPK is localized in the actin belt, and is important for the mature osteoclast formation. Taken together, we demonstrate here the critical role of hnRNPK in osteoclast differentiation, and depict a model in which the cytoplasmic hnRNPK interacts with GSK3β and regulates its function.

NF-κB-Mediated Regulation of Osteoclastogenesis

Osteoclasts are multinucleated cells formed mainly on bone surfaces in response to cytokines by fusion of bone marrow-derived myeloid lineage precursors that circulate in the blood. Major advances in understanding of the molecular mechanisms regulating osteoclast formation and functions have been made in the past 20 years since the discovery that their formation requires nuclear factor-κB (NF-κB) signaling and that this is activated in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which also controls osteoclast activation to resorb (degrade) bone. These studies have revealed that RANKL and some pro-inflammatory cytokines, including tumor necrosis factor, activate NF-κB and downstream signaling, including c-Fos and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), and inhibition of repressors of NFATc1 signaling, to positively regulate osteoclast formation and functions. However, these cytokines also activate NF-κB signaling that can limit osteoclast formation through the NF-κB signaling proteins, TRAF3 and p100, and the suppressors of c-Fos/NFATc1 signaling, IRF8, and RBP-J. This paper reviews current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast formation and activation.

Osteopetrosis in TAK1-deficient mice owing to defective NF-κB and NOTCH signaling

The MAP kinase TGFβ-activated kinase (TAK1) plays a crucial role in physiologic and pathologic cellular functions including cell survival, differentiation, apoptosis, inflammation, and oncogenesis. However, the entire repertoire of its mechanism of action has not been elucidated. Here, we found that ablation of Tak1 in myeloid cells causes osteopetrosis in mice as a result of defective osteoclastogenesis. Mechanistically, Tak1 deficiency correlated with increased NUMB-like (NUMBL) levels. Accordingly, forced expression of Numbl abrogated osteoclastogenesis whereas its deletion partially restored osteoclastogenesis and reversed the phenotype of Tak1 deficiency. Tak1 deletion also down-regulated Notch intracellular domain (NICD), but increased the levels of the transcription factor recombinant recognition sequence binding protein at Jκ site (RBPJ), consistent with NUMBL regulating notch signaling through degradation of NICD, a modulator of RBPJ. Accordingly, deletion of Rbpj partially corrected osteopetrosis in Tak1-deficient mice. Furthermore, expression of active IKK2 in RBPJ/TAK1-deficient cells significantly restored osteoclastogenesis, indicating that activation of NF-κB is essential for complete rescue of the pathway. Thus, we propose that TAK1 regulates osteoclastogenesis by integrating activation of NF-κB and derepression of NOTCH/RBPJ in myeloid cells through inhibition of NUMBL.

Resistin stimulates expression of chemokine genes in chondrocytes via combinatorial regulation of C/EBPβ and NF-κB

To further investigate the regulation role of two chemokine genes CCL3 and CCL4 in chondrocytes in response to resistin, human primary chondrocytes and T/C-28a2 cells were cultured. The function of resistin on the chemokine genes, and the expression of C/EBPβ, NF-κB isoforms were tested using qPCR. The methods used to investigate timed co-regulation of C/EBPβ and NF-κB were NF-κB inhibitor (IKK-NBD) and C/EBPβ inhibitor (SB303580) treatments, and subcellular localization, with or without resistin stimulation. Results showed that resistin could increase the up-regulation of chemokine genes independently. Resistin increased the expression of C/EBPβ and NF-κB isoforms. C/EBPβ regulated basal activity and steadily increased over time up to 24h with resistin. NF-κB was up-regulated upon induction with resistin, peaking at 4 h. C/EBPβ and NF-κB co-enhanced the chemokines expression; inhibition of their activity was additive. The timing of activation in chondrocytes was confirmed by subcellular localization of C/EBPβ and c-rel. Chondrocytes react to resistin in a non-restricted cell-specific manner, utilizing C/EBPβ and NF-κB in a combinatorial regulation of chemokine gene expression. The activity of C/EBPβ is augmented by a transient increase in activity of NF-κB, and both transcription factors act independently on the chemokine genes, CCL3 and CCL4. Thus, resistin stimulates CCL3 and CCL4 through combinatorial regulation of C/EBPβ and NF-κB in chondrocytes.

Emodin regulates bone remodeling by inhibiting osteoclastogenesis and stimulating osteoblast formation

Bone remodeling, a physiological process in which new bone is formed by osteoblasts and the preexisting bone matrix is resorbed by osteoclasts, is vital for the maintenance of healthy bone tissue in adult humans. Imbalances in this process can cause various pathological conditions, including osteoporosis. Emodin, a naturally occurring anthraquinone derivative found in Asian herbal medicines, has numerous beneficial pharmacologic effects, including anticancer and antidiabetic activities. However, the effect of emodin on the regulation of osteoblast and osteoclast activity has not yet been investigated. We show here that emodin is a potential target for osteoporosis therapeutics, as treatment with this agent enhances osteoblast differentiation and bone growth and suppresses osteoclast differentiation and bone resorption. In this study, emodin suppressed receptor activator of nuclear factor-κB (NF-κB) ligand (RANKL)-induced osteoclast differentiation of bone marrow macrophages (BMMs) and the bone-resorbing activity of mature osteoclasts by inhibiting RANKL-induced NF-κB, c-Fos, and NFATc1 expression. Emodin also increased ALP, Alizarin Red-mineralization activity, and the expression of osteoblastogenic gene markers, such as Runx2, osteocalcin (OCN), and ALP in mouse calvarial primary osteoblasts, as well as activated the p38-Runx2 pathway, which enhanced osteoblast differentiation. Moreover, mice treated with emodin showed marked attenuation of lipopolysaccharide (LPS)-induced bone erosion and increased bone-forming activity in a mouse calvarial bone formation model based on micro-computed tomography and histologic analysis of femurs. Our findings reveal a novel function for emodin in bone remodeling, and highlight its potential for use as a therapeutic agent in the treatment of osteoporosis that promotes bone anabolic activity and inhibits osteoclast differentiation.

Constitutive activation of IKK2/NF-κB impairs osteogenesis and skeletal development

Pathologic conditions impair bone homeostasis. The transcription factor NF-κB regulates bone homeostasis and is central to bone pathologies. Whereas contribution of NF-κB to heightened osteoclast activity is well-documented, the mechanisms underlying NF-κB impact on chondrocytes and osteoblasts are scarce. In this study, we examined the effect of constitutively active IKK2 (IKK2ca) on chondrogenic and osteogenic differentiation. We show that retroviral IKK2ca but not GFP, IKK2WT, or the inactive IKK2 forms IKK2KM and IKK2SSAA, strongly suppressed osteogenesis and chondrogenesis, in vitro. In order to explore the effect of constitutive NF-κB activation on bone formation in vivo, we activated this pathway in a conditional fashion. Specifically, we crossed the R26StopIKK2ca mice with mice carrying the Col2-cre in order to express IKK2ca in osteoblasts and chondrocytes. Both chondrocytes and osteoblasts derived from Col2Cre/IKK2ca expressed IKK2ca. Mice were born alive yet died shortly thereafter. Histologically, newborn Col2Cre+/RosaIKK2ca heterozygotes (Cre+IKK2ca_w/f (het)) and homozygotes (Cre+IKK2ca_f/f (KI)) showed smaller skeleton, deformed vertebrate and reduced or missing digit ossification. The width of neural arches, as well as ossification in vertebral bodies of Cre+IKK2ca_w/f and Cre+IKK2ca_f/f, was reduced or diminished. H&E staining of proximal tibia from new born pups revealed that Cre+IKK2ca_f/f displayed disorganized hypertrophic zones within the smaller epiphysis. Micro-CT analysis indicated that 4-wk old Cre+IKK2ca_w/f has abnormal trabecular bone in proximal tibia compared to WT littermates. Mechanistically, ex-vivo experiments showed that expression of differentiation markers in calvarial osteoblasts derived from newborn IKK2ca knock-in mice was diminished compared to WT-derived cells. In situ hybridization studies demonstrated that the hypertrophic chondrocyte marker type-X collagen, the pre-hypertrophic chondrocyte markers Indian hedgehog and alkaline phosphatase, and the early markers Aggrecan and type-II collagen were reduced in Cre+IKK2ca_w/f and Cre+IKK2ca_f/f mice. Altogether, the in-vitro, in vivo and ex-vivo evidence suggest that IKK2ca perturbs osteoblast and chondrocyte maturation and impairs skeletal development.

Anti-cancer IAP antagonists promote bone metastasis: a cautionary tale

The bone microenvironment is complex, containing bone-forming osteoblasts, bone-resorbing osteoclasts, bone-maintaining osteocytes, hematopoietic lineage cells, as well as blood vessels, nerves, and stromal cells. Release of embedded growth factors from the bone matrix via osteoclast resorption has been shown to participate in the alteration of bone microenvironment to facilitate tumor metastasis to this organ. Many types of malignancies including solid tumors and leukemias are associated with elevated levels of inhibitor of apoptosis (IAP) proteins, and IAP antagonists represent an important emerging class of anti-cancer agents. IAPs exert anti-apoptotic roles by inhibiting caspases and upregulating pro-survival proteins, at least in part by activating classical NF-κB signaling. In addition, IAPs act as negative regulators in the alternative NF-κB pathway, so that IAP antagonists stimulate this pathway. The role of the classical NF-κB pathway in IAP antagonist-induced apoptosis has been extensively studied, whereas much less attention has been paid to the role of these agents in the alternative pathway. Thus far, several IAP antagonists have been tested in preclinical and early stage clinical trials, and have shown promise in sensitizing tumor cells to apoptosis without significant side effects. However, recent preclinical evidence suggests an increased risk of bone metastasis caused by IAP antagonists, along with potential for promoting osteoporosis. In this review, the connection between IAP antagonists, the alternative NF-κB pathway, osteoclasts, and bone metastasis are discussed. In light of these effects of IAP antagonists on the bone microenvironment, more attention should be paid to this and other host tissues as these drugs are developed further.

NF-κB signaling and bone resorption

The transcription factor NF-κB is a family of proteins involved in signaling pathways essential for normal cellular functions and development. Deletion of various components of this pathway resulted with abnormal skeletal development. Research in the last decade has established that NF-κB signaling mediates RANK ligand-induced osteoclastogenesis. Consistently, it was shown that inhibition of NF-κB was an effective approach to inhibit osteoclast formation and bone resorptive activity. Identification of the molecular machinery underlying NF-κB activation permitted osteoclast-specific deletion of the major components of this pathway. As a result, it was clear that deletion of members of the proximal IKK kinase complex and the distal NF-κB subunits and downstream regulators affected skeletal development. These studies provided several targets of therapeutic intervention in osteolytic diseases. NF-κB activity has been also described as the centerpiece of inflammatory responses and is considered a potent mediator of inflammatory osteolysis. Indeed, inflammatory insults exacerbate physiologic RANKL-induced NF-κB signals leading to exaggerated responses and to inflammatory osteolysis. These superimposed NF-κB activities appear to underlie several bone pathologies. This review will describe the individual roles of NF-κB molecules in bone resorption and inflammatory osteolysis.

Ubiquitin-like domain of IKKβ regulates osteoclastogenesis and osteolysis

The transcription factor NF-κB family is central for osteoclastogenesis and inflammatory osteolysis. Activation of NF-κB dimers is regulated by a kinase complex predominantly containing IKKα (IKK1), IKKβ (IKK2), and a regulatory subunit, IKKγ/NEMO. IKKα and IKKβ catalyze the cytoplasmic liberation and nuclear translocation of various NF-κB subunits. The requirement of IKKα and IKKβ for normal bone homeostasis has been established. Congruently, mice devoid of IKKα or IKKβ exhibit in vitro and in vivo defects in osteoclastogenesis, and IKKβ-null mice are refractory to inflammatory arthritis and osteolysis. To better understand the molecular mechanism underlying IKKβ function in bone homeostasis and bone pathologies, we conducted structure-function analysis to determine IKKβ functional domains in osteoclasts. IKKβ encompasses several domains, of which the ubiquitination-like domain (ULD) has been shown essential for IKKβ activation. In this study, we examined the role of ULD in IKKβ-mediated NF-κB activation in osteoclast precursors and its contribution to osteoclastogenesis and osteolysis. We generated and virally introduced IKKβ in which the ULD domain has been deleted (IKKβ∆ULD) into osteoclast progenitors. The results show that deletion of ULD diminishes IKKβ activity and that IKKβ∆ULD strongly inhibits osteoclastogenesis. In addition, unlike wild type (WT)-IKKβ, IKKβ∆ULD fail to restore RANKL-induced osteoclastogenesis by IKKβ-null precursors. Finally, we provide evidence that IKKβ∆ULD blocks inflammatory osteolysis in a model of murine calvarial osteolysis. Thus, we identified the ULD as crucial for IKKβ activity and osteoclastogenesis and found that ULD-deficient IKKβ is a potent inhibitor of osteoclastogenesis and osteolysis.

Advances in osteoclast biology reveal potential new drug targets and new roles for osteoclasts

Osteoclasts are multinucleated myeloid lineage cells formed in response to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) by fusion of bone marrow-derived precursors that circulate in the blood and are attracted to sites of bone resorption in response to factors, such as sphingosine-1 phosphate signaling. Major advances in understanding of the molecular mechanisms regulating osteoclast functions have been made in the past 20 years, mainly from mouse and human genetic studies. These have revealed that osteoclasts express and respond to proinflammatory and anti-inflammatory cytokines. Some of these cytokines activate NF-κB and nuclear factor of activated T cells, cytoplasmic 1 (NFATc1) signaling to induce osteoclast formation and activity and also regulate communication with neighboring cells through signaling proteins, including ephrins and semaphorins. Osteoclasts also positively and negatively regulate immune responses and osteoblastic bone formation. These advances have led to development of new inhibitors of bone resorption that are in clinical use or in clinical trials; and more should follow, based on these advances. This article reviews current understanding of how bone resorption is regulated both positively and negatively in normal and pathologic states.

Involvement of kinase PKC-zeta in the p62/p62(P392L)-driven activation of NF-κB in human osteoclasts

Mutations of the gene encoding sequestosome1 (SQSTM1/p62), clustering in or near the UBA domain, have been described in Paget's disease of bone (PDB); among these the P392L substitution is the most prevalent. Protein p62 mediates several cell functions, including the control of NF-κB signaling, and autophagy. This scaffolding protein interacts with atypical PKCζ in the RANKL-induced signaling complex. We have previously shown that osteoclasts (OCs) overexpressing the p62(P392L) variant were in a constitutively activated state, presenting activated kinase p-PKCζ/λ and activated NF-κB prior to RANKL stimulation. In the present study, we investigated the relationships between PKCζ and NF-κB activation in human OCs transfected with p62 variants. We showed that PKCζ and p-PKCζ/λ co-localize with p62, and that PKCζ is involved in the RANKL-induced NF-κB activation and in the RANKL-independent activation of NF-κB observed in p62(P392L)-transfected cells. We also observed a basal and RANKL-induced increase in IκBα levels in the presence of the p62(P392L) mutation that contrasted with the NF-κB activation. In this study we propose that PKCζ plays a role in the activation of NF-κB by acting as a p65 (RelA) kinase at Ser(536), independently of IκBα; this alternative pathway could be used preferentially in the presence of the p62(P392L) mutation, which may hinder the ubiquitin-proteasome pathway. Overall, our results highlight the importance of p62-associated PKCζ in the overactive state of pagetic OCs and in the activation of NF-κB, particularly in the presence of the p62(P392L) mutation.

Bone loss biomarkers associated with peri-implantitis. A cross-sectional study

AIM

To investigate the levels of biomarkers associated with osteoclastogenesis in patients suffering peri-implantitis and to compare them with levels in healthy peri-implant sites and severe chronic periodontitis.

MATERIAL AND METHODS

Peri-implant/gingival crevicular fluid samples and clinical parameters including: bleeding on probing, modified Plaque Index (PlI), pocket depth and clinical attachment level were collected from 70 patients (23 with peri-implantitis, 25 with healthy peri-implant tissues and 22 with severe chronic periodontitis). The concentrations of sRANKL, RANK and OPG were evaluated using enzyme-linked immunosorbent assays; they were compared between the groups and correlated with the clinical findings.

RESULTS

sRANKL (P = 0.01), RANK (P = 0.01) and OPG (P = 0.03) concentrations were significantly higher in peri-implantitis sites when compared to those in healthy implant sites, although differences in the sRANKL/OPG ratio were not statistically significant. In these sites all three markers were significantly correlated with the clinical parameters, with exception of OPG/PI correlation that remained insignificant (P = 0.121). When comparing peri-implantitis and periodontitis findings, RANK was significantly higher in peri-implantitis sites whereas, sRANKL (P = 0.03) and sRANKL/OPG ratio (P = 0.004) were significantly higher in periodontitis sites. Among periodontitis and healthy implant sites the same differences have been observed for both sRANKL (P = 0.000) and sRANKL/OPG ratio (P = 0.000), furthermore RANK was higher in periodontitis sites as well (P = 0.010).

CONCLUSION

The findings of this preliminary study on a relatively small sample size suggest that the PICF levels of biomarkers sRANKL, RANK, and OPG are associated with peri-implant tissue destruction and the pattern of these biomarkers differed when compared to periodontitis.

Constitutively active canonical NF-κB pathway induces severe bone loss in mice

Physiologic osteoclastogenesis entails activation of multiple signal transduction pathways distal to the cell membrane receptor RANK. However, atypical osteoclastogenesis driven by pro-inflammatory stimuli has been described. We have reported recently a novel mechanism whereby endogenous mutational activation of the classical NF-κB pathway is sufficient to induce RANKL/RANK-independent osteoclastogenesis. Here we investigate the physiologic relevance of this phenomenon in vivo. Using a knock-in approach, the active form of IKK2, namely IKK2SSEE, was introduced into the myeloid lineage with the aid of CD11b-cre mice. Phenotypic assessment revealed that expression of IKK2SSEE in the myeloid compartment induced significant bone loss in vivo. This observation was supported by a dramatic increase in the number and size of osteoclasts in trabecular regions, elevated levels of circulating TRACP-5b, and reduced bone volume. Mechanistically, we observed that IKK2SSEE induced high expression of not only p65 but also p52 and RelB; the latter two molecules are considered exclusive members of the alternative NF-κB pathway. Intriguingly, RelB and P52 were both required to mediate the osteoclastogenic effect of IKK2SSEE and co-expression of these two proteins was sufficient to recapitulate osteoclastogenesis in the absence of RANKL or IKK2SSEE. Furthermore, we found that NF-κB2/p100 is a potent inhibitor of IKK2SSEE-induced osteoclastogenesis. Deletion of p52 enabled more robust osteoclast formation by the active kinase. In summary, molecular activation of IKK2 may play a role in conditions of pathologic bone destruction, which may be refractory to therapeutic interventions targeting the proximal RANKL/RANK signal.

Inhibitory regulation of osteoclast differentiation by interleukin-3 via regulation of c-Fos and Id protein expression

Interleukin-3 (IL-3) is produced under various pathological conditions and is thought to be involved in the pathogenesis of inflammatory diseases; however, its function in bone homeostasis under normal conditions or nature of the downstream molecular targets remains unknown. Here we examined the effect of IL-3 on osteoclast differentiation from mouse and human bone marrow-derived macrophages (BMMs). Although IL-3 can induce osteoclast differentiation of multiple myeloma bone marrow cells, IL-3 greatly inhibited osteoclast differentiation of human BMMs isolated from healthy donors. These inhibitory effects of IL-3 were only observed at early time points (days 0 and 1). IL-3 inhibited the expression of c-Fos and NFATc1 in BMMs treated with RANKL. However, IL-3-mediated inhibition of osteoclast differentiation was not completely reversed by ectopic expression of c-Fos or NFATc1. Importantly, IL-3 induced inhibitor of DNA binding/differentiation (Id)1 in hBMMs, while Id2 were sustained during osteoclast differentiation of mBMMs treated with IL-3. Ectopic expression of NFATc1 in Id2-deficient BMMs completely reversed the inhibitory effect of IL-3 on osteoclast differentiation. Furthermore, inflammation-induced bone erosion was markedly inhibited by IL-3 administration. Taken together, our results suggest that IL-3 plays an inhibitory role in osteoclast differentiation by regulating c-Fos and Ids, and also exerts anti-bone erosion effects.

Evidence that the kinase-truncated c-Src regulates NF-κB signaling by targeting NEMO

The tyrosine kinase c-Src and transcription factor NF-κB are considered crucial components required for normal osteoclastogenesis. Genetic ablation of either pathway leads to detrimental osteopetrotic phenotypes in mice. Similarly, obstruction of either pathway halts osteoclastogenesis and lessens various forms of bone loss. It has been shown previously that mice expressing a kinase domain-truncated c-Src, termed Src251, develop severe osteopetrosis owing to increased osteoclast apoptosis. It was further suggested that this phenomenon is associated with reduced Akt kinase activity. However, the precise mechanism underlying the osteoclast inhibitory effect of Src251 remains obscure. C-Src associates with TRAF6-p62 interacting with receptor activator of NF-κB (RANK) distal region and the complex facilitate activation of RANK down stream signal transduction cascades including NF-κB. Given this proximity between c-Src and NF-κB signaling in osteoclasts, we surmised that inhibition of osteoclastogenesis by Src251 may be achieved through inhibition of NF-κB signaling. We have demonstrated recently that NEMO, the regulatory subunit of the IKK complex, is crucial for osteoclastogenesis and interacts with c-Src in osteoclast progenitors. Transfection studies, in which we employed various forms of c-Src and NEMO, revealed that the dominant negative form of c-Src, namely Src251, mediates degradation of NEMO thus halting NF-κB signaling. Furthermore, degradation of NEMO requires its intact zinc finger domain which is located at the ubiquitination domain. This process also requires appropriate cellular localization of Src251, since deletion of its myristoylation domain ablates its degradation capacity. Buttressing these findings, the expression of NEMO and NF-κB signaling were significantly reduced in monocytes collected from Src251 transgenic mice.

Polyubiquitination events mediate polymethylmethacrylate (PMMA) particle activation of NF-kappaB pathway

The pathologic response to implant wear-debris constitutes a major component of inflammatory osteolysis and remains under intense investigation. Polymethylmethacrylate (PMMA) particles, which are released during implant wear and loosening, constitute a major culprit by virtue of inducing inflammatory and osteolytic responses by macrophages and osteoclasts, respectively. Recent work by several groups has identified important cellular entities and secreted factors that contribute to inflammatory osteolysis. In previous work, we have shown that PMMA particles contribute to inflammatory osteolysis through stimulation of major pathways in monocytes/macrophages, primarily NF-κB and MAP kinases. The former pathway requires assembly of large IKK complex encompassing IKK1, IKK2, and IKKγ/NEMO. We have shown recently that interfering with the NF-κB and MAPK activation pathways, through introduction of inhibitors and decoy molecules, impedes PMMA-induced inflammation and osteolysis in mouse models of experimental calvarial osteolysis and inflammatory arthritis. In this study, we report that PMMA particles activate the upstream transforming growth factor β-activated kinase-1 (TAK1), which is a key regulator of signal transduction cascades leading to activation of NF-κB and AP-1 factors. More importantly, we found that PMMA particles induce TAK1 binding to NEMO and UBC13. In addition, we show that PMMA particles induce TRAF6 and UBC13 binding to NEMO and that lack of TRAF6 significantly attenuates NEMO ubiquitination. Altogether, these observations suggest that PMMA particles induce ubiquitination of NEMO, an event likely mediated by TRAF6, TAK1, and UBC13. Our findings provide important information for better understanding of the mechanisms underlying PMMA particle-induced inflammatory responses.

Role of NF-κB in the skeleton

Since the discovery that deletion of the NF-κB subunits p50 and p52 causes osteopetrosis in mice, there has been considerable interest in the role of NF-κB signaling in bone. NF-κB controls the differentiation or activity of the major skeletal cell types - osteoclasts, osteoblasts, osteocytes and chondrocytes. However, with five NF-κB subunits and two distinct activation pathways, not all NF-κB signals lead to the same physiologic responses. In this review, we will describe the roles of various NF-κB proteins in basal bone homeostasis and disease states, and explore how NF-κB inhibition might be utilized therapeutically.

CCAAT/enhancer-binding protein β and NF-κB mediate high level expression of chemokine genes CCL3 and CCL4 by human chondrocytes in response to IL-1β

A large set of chemokines is highly up-regulated in human chondrocytes in response to IL-1β (Sandell, L. J., Xing, X., Franz, C., Davies, S., Chang, L. W., and Patra, D. (2008) Osteoarthr. Cartil. 16, 1560-1571). To investigate the mechanism of transcriptional regulation, deletion constructs of selected chemokine gene promoters, the human CCL3 (MIP-1α) and CCL4 (MIP-1β), were transfected into human chondrocytes with or without IL-1β. The results show that an IL-1β-responsive element is located between bp -300 and -140 of the CCL3 promoter and between bp -222 and -100 of the CCL4 promoter. Because both of these elements contain CCAAT/enhancer-binding protein β (C/EBPβ) motifs, the function of C/EBPβ was examined. IL-1β stimulated the expression of C/EBPβ, and the direct binding of C/EBPβ to the C/EBPβ motif was confirmed by EMSA and ChIP analyses. The -300 bp CCL3 promoter and -222 bp CCL4 promoter were strongly up-regulated by co-transfection with the C/EBPβ expression vector. Mutation of the C/EBPβ motif and reduction of C/EBPβ expression by siRNA decreased the up-regulation. Additionally, another cytokine-related transcription factor, NF-κB, was also shown to be involved in the up-regulation of chemokines in response to IL-1β, and the binding site was identified. The regulation of C/EBPβ and NF-κB was confirmed by the inhibition by C/EBPβ and NF-κB and by transfection with C/EBPβ and NF-κB expression vectors in the presence or absence of IL-1β. Taken together, our results suggest that C/EBPβ and NF-κB are both involved in the IL-1β-responsive up-regulation of chemokine genes in human chondrocytes. Time course experiments indicated that C/EBPβ gradually and steadily induces chemokine up-regulation, whereas NF-κB activity was highest at the early stage of chemokine up-regulation.

Tyrosine phosphorylation is required for IkappaB kinase-beta (IKKbeta) activation and function in osteoclastogenesis

The transcription factor NF-kappaB is crucial for numerous cellular functions such as survival, differentiation, immunity, and inflammation. A key function of this family of transcription factors is regulation of osteoclast differentiation and function, which in turn controls skeletal homeostasis. The IkappaB kinase (IKK) complex, which contains IKKalpha, IKKbeta, and IKKgamma, is required for activation of NF-kappaB, and deletion of either IKKalpha or IKKbeta resulted with defective osteoclast differentiation and survival. We have recently investigated the details of the mechanisms governing the role of IKKbeta in osteoclastogenesis and found that constitutively active IKKbeta in which serine residues 177/181 were mutated into negatively charged glutamic acids instigates spontaneous bona fide receptor activator of NF-kappaB ligand (RANKL)-independent osteoclastogenesis. To better understand and define the functional role of IKKbeta domains capable of regulating the osteoclastogenic activity of IKK, we investigated key motifs in the activation T loop of IKKbeta, which are potentially capable of modulating its osteoclastogenic activity. We discovered that dual serine (traditional serine residues 177/181) and tyrosine (188/199) phosphorylation events are crucial for IKKbeta activation. Mutation of the latter tyrosine residues blunted the NF-kappaB activity of wild type and constitutively active IKKbeta, and tyrosine 188/199-deficient IKKbeta inhibited osteoclastogenesis. Thus, tyrosines 188/199 are a novel target for regulating IKKbeta activity, at least in osteoclasts.